Infrared Space Observatory (ISO)

The Infrared Space Observatory (ISO) was an astronomical satellite launched by the European Space Agency (ESA) in 1995. The main feature of the ISO was a telescope that studied radiation from the infrared range of the electromagnetic spectrum. Because Earth's atmosphere filters out most infrared radiation, the ISO was able to get a more complete picture of the cosmos than could be obtained from the ground. The observatory remained operational for about three years before it was shut down and purposely burned up in the atmosphere. In that time, the ISO was able to detect ice and water vapor around distant stars and interstellar gas clouds. It was also able to provide scientists with a look back in time with information on how the galaxies formed.

Background

The ISO operated in a band of the electromagnetic spectrum called the infrared. The electromagnetic spectrum is a form of radiation divided into sections according to wavelength and frequency. On one end are low-wavelength, high-frequency gamma rays; on the other end are high-wavelength, low-frequency radio waves. The light visible to the human eye falls in a narrow band of the spectrum, sandwiched between ultraviolet and infrared. Infrared is invisible to humans but can be detected in the form of heat.

Most objects in the universe emit radiation from every part of the electromagnetic spectrum. In astronomy, telescopes that focus on a specific band of this spectrum can sometimes provide insight on an object that would not have been possible through visible light alone. Infrared telescopes, for example, can detect the heat energy given off by galaxies, gas clouds, and stars, and reveal details of those objects not possible in visible light. The instruments can also penetrate through interstellar dust that obscures some visual observations, and can also discover objects too cool to emit visible light. Because Earth's atmosphere blocks or absorbs most bands of the electromagnetic spectrum, including infrared, it limits the picture of the universe available from ground-based telescopes. The planet, the atmosphere, and the telescope itself also emit heat, making observations from Earth more difficult. As a result, most infrared astronomy is conducted by satellites orbiting above the atmosphere.

Overview

The first infrared satellite was a joint project between the United States, Britain, and the Netherlands, launched in 1983. The Infrared Astronomical Satellite (IRAS) required a significant amount of coolant to operate, which limited its life span to ten months. In that time, it mapped about 96 percent of the sky and gave scientists one of the first glimpses of the center of the Milky Way galaxy. The success of IRAS prompted officials at the European Space Agency to propose building their own infrared satellite—a project that became the Infrared Space Observatory.

The ISO was launched on November 17, 1995, from the ESA's Guiana Space Centre in Kourou, French Guiana. The craft was around 17 feet long, 12 feet wide, and weighed nearly 5,290 pounds. It orbited Earth once every twenty-four hours, ranging from a distance of approximately 620 miles to 43,800 miles. The ISO's highly elongated orbit brought it within Earth's radiation belts for about seven hours a day. During this time, the craft's instruments were unable to function, so mission control turned them off.

The observatory had a telescope with a 24-inch mirror used to focus light onto four infrared-monitoring instruments. These included a high-resolution camera called ISOCAM, which operated similar to a conventional camera, except within the infrared band of the spectrum. The ISOPHOT, or ISO photo-polarimeter, measured infrared radiation. The short-wave spectrometer (SWS) and long-wave spectrometer (LWS) were devices used to record the chemical compositions of astronomical objects. Both operated in different regions of the infrared spectrum, with the LWS focusing on cooler objects. The ISO's instruments were capable of monitoring the infrared spectrum between a wavelength range of 2.5 to 240 microns. A micron is a unit of measurement corresponding to one-millionth of a meter.

To enable the highly sensitive equipment to function, the instruments had to be kept at a temperature approaching absolute zero—minus 459.67 degrees Fahrenheit. In theory, absolute zero is the point at which all molecular activity stops and is the coldest temperature possible. To maintain the supercooled temperatures necessary, the ISO was also equipped with a cryostat, a device filled with about six hundred gallons of liquid helium. The cryostat had a limited amount of helium, giving the satellite an estimated life span of about eighteen months. The ISO outlived expectations and was able to record observations until it ran out of helium fuel in April of 1998—about a year longer than planned. It was turned off on May 16 and sent on a trajectory to burn up in Earth's atmosphere so it would not become a piece of orbiting "space junk."

During its almost thirty-month lifespan, the ISO orbited Earth more than nine hundred times and made more than thirty thousand scientific observations. It detected the presence of ice and water vapor in the atmospheres of planets within the solar system and in the gas clouds surrounding distant stars. The ISO was able to see through clouds of obscuring dust to discover new stars in the process of being formed and found traces of deadly hydrogen cyanide in the gas surrounding some newborn stars. The observatory also gave astronomers new insight into the earliest days of galaxy formation. Since light from some distant galaxies takes millions or billions of years to reach Earth, the ISO's observations presented a snapshot of the universe as it was almost ten billion years ago. It showed early galaxies producing stars at a rate faster than previously thought.

The ISO's success set the stage for future missions to study the infrared universe. The National Aeronautics and Space Administration (NASA) in the United States launched the Spitzer Space Telescope in 2003. It transmitted data until 2009 when it ran out of coolant and was shut down. That same year, the ESA launched the Herschel Space Observatory, which operated until 2013. NASA launched the James Webb Space Telescope, a next-generation tool in infrared astronomy, in 2021.

Bibliography

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